Metal Slinger (Metal & Flame Book 1) by Rachel Schneider

Sail the seas. Master the blade. Protect your heart.
 
What would you do to have it all?
 
Even though she's not one of them, Brynn has spent her life among the Alaha - a community exiled to live at sea - training to be a guard and waiting for the chance to attend the annual market hosted by the Kenta - the very same people who exiled her adopted community. Going to the market is a rite of passage eagerly anticipated by all young guards, but Brynn does not anticipate breaking a century-long peace treaty while there... Or for an intense encounter with an enemy soldier to threaten the fragile peace between their people and everything Brynn once believed to be true.
 
Soon, Brynn's loyalty to the Alaha is tested when an unexpected visitor tracks her down - none other than said enemy soldier - sworn under oath to awaken Brynn to her true identity. Sent to sea and feeling more alone than ever, there a world of denied magic is unveiled. With rumours of a growing rebellion, Brynn's future has never looked so unsure. She was once certain of her fate and where she belonged, but the dark, knowing eyes of this stranger have her questioning everything, including her heart...